Sciencemadness Discussion Board - Boiling Down Sulfuric Acid Problems

Boiling Down Sulfuric Acid Problems

K12Chemistry - 13-2-2013 at 13:27

I know there are a lot of threads that already mention this but AFAIK there aren't any that answer these.

1. I need to boil down sulfuric acid that I made electrochemically. The problem is I don't know the concentration of the acid so I don't know when to stop boiling. Is there any way that I can know when it reaches 95%+?

2. When I finish boiling down the sulfuric acid when it is 98%, I don't want to handle it when it is hot. Since it is hygroscopic by the time it cools down do you think that it will have lost a significant amount of concentration?

3. I just ordered a hot plate so that I don't risk the glass breaking by using a direct flame. So should I boil it down in a beaker directly on the hot plate. Or should I use a round bottom flask with a sand bath? The thing about the sand bath is it will take a long time to cool down so the sulfuric acid might dilute itself.

4. What are the risks in doing this, I know it releases a lot of SO3 and SO2 vapours. I am NOT a qualified chemist with training so is it safe if I do it outside with proper glassware, face shield, thick gloves and a lab coat?

5. How can I store the concentrated sulfuric acid? Would this container be safe:
http://www.philipharris.co.uk/product/Glass-Bottles-125mL-B8...
What specifications are sufficient to safely store sulfuric acid?

6. Are boiling stones/chips necessary or would you recommend them?

hissingnoise - 13-2-2013 at 13:52

Sulphuric acid evolves some SO₃ as a thick, white vapour at its boiling point (337°C)!
When this heavy fuming is noticed, conc. will be ~98%!
I did my boiling in a shallow, pyrex cooking dish in a well-ventilated shed . . .
Today's pyrex dishes may not be borosilicate and may fail and crack on heating!

blogfast25 - 13-2-2013 at 13:52

Trying to obtain concentrated sulphuric acid from weak acid by simple boiling in is a slow process and you'll start losing SO3 from about 60 % onwards.

One way of checking concentration is by means of density (a suitable hydrometer will do it). 95 % H2SO4 is about 1.833 specific gravity. There are tables that show concentration of acid v. density accurately.

Electrical plate is much preferred over direct flame. Make sure you're prepared for any spill or breakage (sit the plate inside a large plastic box, e.g., that would contain any serious spills).

Storage: in glass or thick HDPE containers.

[Edited on 13-2-2013 by blogfast25]

Endimion17 - 13-2-2013 at 14:02

You could've just asked right in those very threads, but ok...

1. You don't boil it down in a beaker, it's too unsafe. Use a distillation apparatus. Once the water is gone, the thermometer will stop registering 100 °C and the temperature will drop down until the acid starts coming over at its, much higher boiling point.

2. If it's in a distillation apparatus, nothing will happen.

3. Do not use a beaker. Use an enclosed system with ground glass joints. Expensive? Yeah, it is. This is an expensive hobby. Boiling sulphuric acid is not something meant to be done in simple vessels.
Use a round bottom flask and shield it with aluminium foil to avoid unneccessary heat loss.

4. If the glass breaks, boiling acid eats skin. Literally. Third degree burns in no time.
No SO2 will evolve, but SO3 will gradually start evolving when the concentration climbs.
Boiling down in a proper apparatus is not something that has to be done outside or in a fume hood if it's properly done, but I doubt you've got the proper working conditions, so you better do it outside.
Face shield is a nice touch. Goggles and a coat are a must, as well as gloves. 3 layers of latex is fine.
Have a bucket of water nearby.

5. It doesn't evaporate, but some of it will reach the bottle's neck when pouring. Use plastic caps supplied with PTFE disc inside. Or store it in a ground glass joint bottle.
The bottle on the link is for storing solids. You need a bottle with narrow mouth. Waxed cardboard discs will fail quickly.

6. You have to use boiling chips. As the water content drops, the acid will boil with difficulty, bumping ver more.

Round bottom flask with boiling chips, attached via ground glass joint to a distillation apparatus, filled half its volume with acid. That's it. No excuses.

BromicAcid - 13-2-2013 at 16:26

Quote: Originally posted by Endimion17

1. You don't boil it down in a beaker, it's too unsafe. Use a distillation apparatus. Once the water is gone, the thermometer will stop registering 100 °C and the temperature will drop down until the acid starts coming over at its, much higher boiling point.

My experience is that as your are distilling water keeps coming off slowly as you heat, not all at once. Hence unless you have a long column water will still be coming off at 250C and that water will not register as a head temp of 100C as it is heated so far above it's boiling point. Better to keep a pot temp and shoot for a target temperature in this specific case.

Quote: Originally posted by Endimion17

6. You have to use boiling chips. As the water content drops, the acid will boil with difficulty, bumping ver more.

Bumping is crazy with sulfuric acid. Boiling chips though, no way, never again! My PTFE boiling chips and by ceramic boiling chips both floated! The PTFE made sense after the fact but ceramic should have had a high enough density. Best for this would be a stir bar or a capillary. Of course this will only be an issue as you get higher concentrations.

My first time concentrating sulfuric acid was actually in a ceramic coated camping pan. It held up and worked fine. I used a coil-hot plate like a stove-top. I did not see any smoke but started to cough after some time. I kept waiting for smoke but didn't see any. I eventually shut it down from the fumes that I could not see. The next day the grass all the yard was dead radiating out from my hotplate. So yes it can be done with minimalist equipment but it's not recommended.

Most of my experience though is from distilling drain cleaner. Now that is some mean stuff.

jock88 - 13-2-2013 at 16:36

Is boiling under a vacuum a good way to concentrate sulphuric acid?

K12Chemistry - 14-2-2013 at 03:36

what do you mean by doing it in distillation apparatus. What kind. Do you mean like the SO3 and water vapour passes down a condenser into another flask or what?

jock88 - 14-2-2013 at 07:37

If you place your water + Sulphuric acid mixture (the stuff you want to concentrate) into a vacuum distillation apparatus, pull a vacuum and heat the flask. Will the water come off at a lower temperature so that you do not have to heat all the way up to the very high temperature needed when doing the job in a simple open vessel.
The wanted product is the stuff remaining behind in the heated vessel.
Something simpler would do instead of the Vacuum distillation apparatus but you would need to have an aspirator pump. The fumes would surly damage a mechanical pump.

Does this work?

[Edited on 14-2-2013 by jock88]

Endimion17 - 14-2-2013 at 09:15

Quote: Originally posted by BromicAcid

I don't think that's water that's coming over at 250 degrees. H2SO4 doesn't form an azeotrope. Water comes over at 100 °C, and if the thermometer registers 250, either it failed, or is positioned in a way that id can be directly heated by thermal radiation and give erroneous values, or SO3 fumes are evolving.
Distillation of two liquids with high boiling point differential, which don't form azeotropes, proceeds in a way that the temperature drops because the fraction at the higher temperature won't immediately follow the one with the lower boiling point. Thermometer just drops because the heat of the whole upper path is being lost to the environment. It's a classic case I've encountered many times in the lab and at home. It also makes me somewhat furious, don't know why.

Quote:

Who the hell uses PTFE and ceramic?

Use glass beads or shards. I make my own boiling chips from the glassware I've broken over the past years. You just heat it, spread it into a capillary and then melt it in the flame. The beads just fall off.

I still don't understand how it is possible to easily obtain strong, concentrated mineral acids in a country that seems to hate home chemistry. I envy you.

Quote: Originally posted by jock88

Is boiling under a vacuum a good way to concentrate sulphuric acid?

Haven't tried it. The bumping would increase dramatically. Pellogio's capillary is the only solution because boiling chips fail in low pressures. Water suckback could create a detonation.
The boiling point should drop, though.

Quote: Originally posted by K12Chemistry

what do you mean by doing it in distillation apparatus. What kind. Do you mean like the SO3 and water vapour passes down a condenser into another flask or what?

Plain old ground glass joint apparatus. It's simple as that.

Magpie - 14-2-2013 at 09:43

Quote: Originally posted by BromicAcid

Steam (water) coming off a boiling concentrated salt solution will be several degrees hotter than 100°C.

Although I have never tried concentrating sulfuric acid (don't have to as I can buy it at the hardware store

) I can see where H2SO4, being non-volatile, would also produce superheated steam.

blogfast25 - 14-2-2013 at 09:53

Here's an article on concentrating H2SO4, 'equipment free'. Jeffrey used to post here and is no fool:

http://amazingrust.com/Experiments/how_to/Concentrating_H2SO...

S.C. Wack - 14-2-2013 at 16:16

That would form big rousing clouds. I have recommended slow in the oven before. Maybe not if your kitchen is all in stainless...there does not "seem" to be "much" S loss unless the door is opened. I suspect 95% is an unobtainable concentration without the proper gear...maybe even from 94% acid.

Magpie - 15-2-2013 at 20:22

The 2nd page of this document shows a plot of the wt%H2SO4 in the liquid vs that in the vapor at equilibrium. So below 77wt% H2SO4 in the liquid the H2SO4 content of the vapor is barely measureable.

http://www.jacobs.com/uploadedfiles/Products/Chemetics/Sulfu...

luckybot - 15-2-2013 at 21:37

So just to verify I understand what has been written; in a ground glass joint setup with column and condenser the procedure would be to just heat the mixture slowly until the temp at the head starts to rise past 100C? And this is going to produce a concentration of close to 98% while also producing minimal fumes?